1. Field of the Invention
This invention relates to a process for the selective preparation of 2-vinylpyridine from acetylene and acrylonitrile with a soluble organocobalt catalyst. As a result of the invention, one can convert acetylene and acrylonitrile selectively, with only moderate catalyst usage, into 2-vinylpyridine.
2. Discussion of the Prior Art
The processes to date for vinylpyridine synthesis are economically unsatisfactory with respect to catalyst utilization and product yields, as is apparent from the data presented in Table 1 which follows.
TABLE 1 __________________________________________________________________________ Pyridine yield Turn over Nitrile Resi- based on Number concen- dence acrylo- mol 2-Vinyl- Liter- Cata- Temp. tration time nitrile pyridine/ ature lyst .degree.C. mols/l min. conversion g-atom Co __________________________________________________________________________ DOS 2G 153 09 P.Hardt Cobalt- 120 4.57 60 41.6% 38 Lonza AG ocene Example 3 H.Yamazaki, J.Wakatsuki Cobalt- 110 5.05 420 31.0% 48.0 Synthesis ocene 1976, 26, page 28 Cyclo- U.S. Pat. No. penta- 4,006,149 dienyl- Studienges. cobalt 100 2.144 240 75.0% 63.0 Kohle mbH cyclo- Example 40 octa- diene __________________________________________________________________________
Yamazaki et al. obtained a 2-vinylpyridine yield of 31% using cobaltocene as catalyst. The synthesis was carried out at 110.degree. C. with a reaction time of 7 hours and an acrylonitrile concentration of 5.05 mols/liter. The reaction was deliberately carried out at a considerably lower temperature, i.e., 110.degree. C., than those employed in the production of other pyridine derivatives in order to reduce the loss of vinylpyridine due to polymerization. According to P. Hardt, a reduction of the reaction time to 60 minutes, using the same catalyst, a slightly higher temperature (120.degree. C.) and a slightly lower nitrile concentration (4.57 mols/l), failed to produce the desired results. The yield was under 45%, which is too low to make the process economically worthwhile.
U.S. Pat. No. 4,006,149, the disclosure of which is hereby specifically incorporated herein by reference, names cobalt (I) complex compounds as catalysts, also for the preparation of vinylpyridines. These catalysts are considerably more effective than cobaltocene, the catalyst of the aforesaid disclosures. At 100.degree. C. and with a residence time of 4 hours and a nitrile concentration of 2.14 mols/l, a yield of 75% is obtained. With the parameters selected as above, a lower nitrile concentration and a moderately long reaction time, it was not likely that a substantial improvement in the result of the synthesis could be obtained by the use of cobaltocene, for example, as catalyst.
Surprisingly, and contrary to all experience, it has now been found that the yield of 2-vinylpyridine susceptible to polymerization can be drastically increased when the temperature is raised to 150.degree. C. and above, the residence time is reduced to less than 50 minutes, and the nitrile concentration is selected at under 2 mols/l. Suitable catalysts for this selective synthesis of 2-vinylpyridine then are cyclopentadienylcobalt compounds or .pi.-allylcobalt compounds generally.
The catalysts are employed in an amount between 0.05-0.5 mol %, preferably 0.1 to 0.3 mol %, based on mols acrylnitrile charged.
With regard to selection of the temperature, the range around 150.degree. to 160.degree. C. is advantageous; however, temperatures ranging from 140.degree. to 180.degree. C. may also be used.
The reaction time may range from about 5 minutes to not more than 50 minutes; however, it may also be under 5 minutes and as short as a few seconds when suitable reaction apparatuses such as flow pipes are used. As a rule, a reaction time between 15 and 30 minutes will be advantageous.
The acrylonitrile concentration may range from about 0.1 mol/l to 2 mols/l or higher. The acrylonitrile concentration is preferably about or above 1 mol/l. The mol ratio of acetylene to acrylnitrile is generally 0.5 to 2:1, preferably 1 to 1.5:1.
Suitable solvents are those which are inert to the reactants over the temperature range specified. Such solvents are aliphatic solvents, for example, and particularly aromatic solvents such as benzene or toluene. Useful aliphatic solvents include: pentane, hexane, commercially available aliphatic mixtures.
The reaction in accordance with the invention is preferably carried out in a pressure vessel equipped with efficient mixing means to provide assurance that the solution is saturated with acetylene during the reaction. The acetylene pressure may range from 5 to 20 atm and is preferably comprised between 8 and 17 atm.
Suitable catalysts for the process in accordance with the invention are temperature-resistant cobalt compounds containing at least one cyclopentadienyl bound to the cobalt, such as bis-cyclopentadienylcobalt (cobaltocene), cyclopentadienylcobalt cyclooctadiene or cyclopentadienylcobalt dicarbonyl, for example, .pi.-allylcobalt compounds can also be used, such as methylheptadienylbutadiene-cobalt, cyclooctenylcyclooctadiene(1,5)-cobalt. Useful catalysts of the above type include all those described or named in U.S. Pat. No. 4,006,149, supra.
The process may be carried out discontinuously or, optionally, continuously, either in cascaded agitator vessels or in a flow pipe, it being important to provide for rapid cooling of the effluent stream. The catalyst is advantageously used in the form of prefabricated cobalt compounds. However, it may also be prepared in situ in the reaction mixture.
The 2-vinylpyridine prepared in accordance with the invention finds use as a component of terpolymeric adhesion promoters in the manufacture of tires and also serves as a comonomer in the production of acrylic fibers.